In many industrial processes which involve heating, the temperature of the material or object(s) undergoing processing is critical to the successful and efficient completion of the process. Accordingly, suitable means are often incorporated in industrial processes to measure temperature. Most of these means (including but not limited to thermocouples, resistance-temperature devices, and so on) require that the measuring instrument be in physical contact with the material whose temperature is to be measured. However, in many situations direct physical contact is either impossible or undesirable. Examples include the processing of paper or plastic sheets where contact will damage the material, processing of caustic or extremely high-temperature materials such as cement where contact with the material will eventually damage the instrument, and the processing of any material on a moving conveyor belt where contact is mechanically difficult to maintain for the time required for a measurement.
It is well known that all objects above 0 degrees Kelvin (0° K.) emit electromagnetic radiation in accordance with Planck's radiation law. At room temperature, the peak intensity of this emission is in the infrared region. Remote measurement of temperature is made possible by this phenomenon through the measurement of the visible or infrared radiation emitted from the material whose temperature is to be measured. Relatively simple and inexpensive commercial instruments for remote measurement of temperature have been developed for this purpose. Virtually all of these instruments use infrared or visible radiation in the wavelength range of 10 microns or shorter.
While such infrared and visible-light remote-temperature measurement devices work well in many applications, there remain a number of industrial circumstances in which these instruments will not work. There are at least three reasons for this. (1) The heating process or related processes produce airborne particulates, condensing vapor, smoke, or other suspended substances which scatter infrared and light rays. (2) The material is being heated with microwave or RF power, often inside a shielded enclosure, and the resulting electromagnetic fields and requisite metallic shielding interfere with the proper operation of the sensor. (3) Because of the nature of the process, the temperature beneath the surface of the material is desired, but infrared and visible-light sensors sense the temperature of most solid objects using only radiation from the surface and a few microns beneath the surface. Because of these and other shortcomings, remote temperature sensing in industrial applications using infrared or visible-light technology cannot be used in a number of situations that could benefit from it.
This invention reduces or eliminates all the above-mentioned problems associated with infrared and visible-light measurement of temperature in the following ways. Because of its longer wavelength, microwave radiation is scattered much less than infrared radiation, so that suspended particles which prevent infrared sensors from operating will present little or no difficulty for microwave remote temperature sensors. A microwave remote temperature sensor can be designed not only to be impervious to the influence of (periodically interrupted) microwave or RF radiation, but can actually take advantage of a shielded enclosure to improve the overall accuracy of the measurement. Depending on the wavelength chosen and the dielectric properties of the object observed, microwave remote sensing of temperature can reveal temperature information from regions as deep as several cm beneath the surface of an object or material.
These advantages derive from the present invention applying these fundamental physical principles in novel ways by using features which make it economical, stable, and reliable, as well as capable of measuring the temperature of objects and materials which at present can either not be measured at all or measured only with great difficulty and expense.
Microwave radiometry has been practiced for scientific and military purposes since before 1945. However, until solid-state devices such as the gallium-arsenide MESFET were recently developed, microwave radiometers were expensive, bulky systems. Recently with the development of high-volume consumer applications of low-noise microwave amplifiers, it is possible to manufacture subsystems such as low-noise block converters (LNBs) that sell for less than $50 US retail.
Experiments with microwave radiometry for industrial applications have been published by several commentators. These researchers have reported the microwave temperature measurement of road asphalt and cement in a kiln. It appears that because of the expense and inconvenience of the equipment used in these experiments, none of the reported radiometers have become available commercially, although some microwave radiometers have been used in laboratory settings for medical purposes.
The typical research-quality microwave radiometer is a complex system consisting of dozens of high-cost microwave components and an equally complex set of low-frequency analog and digital electronics. For reasons of stability, the entire microwave portion of the system is often installed inside an insulated box that is maintained at a constant temperature. One commentator has described several microwave radiometer configurations that have proved to be practical for such scientific uses. These include the simplest “total-power” type and the more sophisticated “Dicke” and “noise-injection” types. The “total-power” type of radiometer is insufficiently stable for industrial use, and the other two types are typically too complex for industrial use. For example, one known noise-injection radiometer uses at least the following microwave components: (1) directional coupler, (2) absorptive PIN-diode switch, (3) noise diode, (4) thermally stabilized reference-temperature load, (5) latching switchable circulator, (6) isolator, (7) LNB and (8) detector.
To address the limitations in the art as described above, there is a need for a substantially simplified form of microwave radiometer that is substantially impervious to changes in system gain or detector characteristics. There is also a need for a radiometer that operates accurately without the need for a temperature-controlled enclosure. A further need is a radiometer with fewer parts.